Human enterovirus D68 (EV-D68) (species, Human enterovirus D; genus, Enterovirus; family, Picornaviridae) can cause severe respiratory tract infections. It was rarely identified in patients in the United States prior to about 2005. However, since the late 2000s, the number of reported EV-D68 cases increased dramatically in various countries. Some EV-D68 infections are characterized by severe disease, requiring intensive care and non-invasive ventilatory support. A 2014 EV-D68 outbreak particularly affected children with a history of asthma or reactive airway disease; and exacerbation of pre-existing asthma or reactive airway disease, similar to that associated with rhinovirus (RV) infection was noted in a high proportion of cases, though some patients with no history of asthma also had asthma-like symptoms (Midgley et al., 2014. MMWR Morb Mortal Wkly Rep 63:798-799).